The present invention is generally related to drilling bits and more particularly involves drill bits having hard metal cutting elements inserted therein, while cutting elements are commonly referred to as inserts and are usually manufactured from a sintered tungsten carbide material. In one embodiment of the invention, the inserts are utilized in a tri-cone rolling cutter drill bit of the type disclosed in U.S. Pat. No. 3,495,668. A similar tri-cone rolling cutter drill bit is disclosed in co-pending application Ser. No. 062,260, filed July 30, 1979, by Kenneth W. Jones for "Oil Well Drilling Bit". In this co-pending application, which is assigned to the assignee of the present application, a tri-cone rolling cutter drill bit is disclosed which utilizes a unique system of insert alignment in the intermediate and gage rows of the cutters.
As mentioned in the aforesaid Jones patent application, the tri-cone bit usually has three rolling cone cutters rotatably mounted on downwardly extending bearing journals at the lower end of the bit body. There are generally two types of rolling-cone cutters utilized in these types of bits: the milled tooth cutter and the insert cutter. The present invention relates to the insert type of bits wherein the cutter cones are made of one material such as a steel alloy, and the cutting elements are made of a harder metal such as a tungsten carbide in a cobalt matrix, and then are pressed into recesses which have been drilled in the cone surfaces. The insert bit offers the advantage of a hard metal cutting element or insert which is tremendously resistant to the abrasive forces normally incurred during drilling operations. Since the inserts are made of a hard material such as tungsten carbide which has been sintered and compacted into a generally cylindrical base portion having a frustoconical protruding portion, the inserts are generally more susceptible to breaking, but on the other hand will outlast a milled tooth cutter several times. The disadvantage of the insert type bits as opposed to the milled tooth bits is that the hard metal inserts are generally not as fracture-resistant as the milled tooth, and therefore cannot be shaped as broad and flat and sharp as the milled tooth. Thus, the bottom hole coverage and penetration rate of the hard metal insert is somewhat less than the milled tooth, although the insert will wear many times longer than the milled tooth.
The conventional insert bits manufactured today generally utilize three rolling cones having circumferential rows of inserts securely attached to the cones by means of interference fit within holes bored substantially perpendicular to the surface of the cone. These conventional cutter cones have rows of inserts placed in circumferential rows on the cone surface in raised shoulders, or lands. The conventional insert type construction suffers from an undesirable effect known as tracking and gyration.
Tracking and gyration occurs because of the circumferential rows of inserts which form grooves in the rock face being drilled. These parallel grooves leave a raised ridge of rock material called a kerf. When this kerf becomes high enough, it causes these rows of inserts to track down grooves cut by the other cutter inserts and results in a drill bit following a noncentral axis of rotation. Thus, the tracking of cutter inserts in the rows formed by adjacent cutters causes a gyration of the bit off the center of rotation of the bit. This orbital gyration is a destructive force on the drill bit, placing high stress on the bearings and cutting structures both. Furthermore, the gyration effect reduces the cutting speed of the bit to a negligible amount, and the resulting kerf buildup eventually contacts the non-cutting surfaces of the cone and almost stops the cutting action of the bit in the formation. The gyration forces introduced during tracking and the orbital motion described above are not those for which the bit is designed, and as a result, unusual and rapid damage occurs to the inserts, the cones and the bearings.
The present invention overcomes these disadvantages by providing a drill bit structure having a unique insert design which reduces failures of the cone structure and greatly reduces gyration and tracking of the conical cutters of the drill bit. The pattern of insert crests on the cutters is a series of sinusoidal circumferential bands in the gage and intermediate rows of the cutter cone surfaces. The aforementioned co-pending application of Kenneth W. Jones discloses a non-linear circumferential row of inserts which is formed by drilling non-linear rows of insert holes in the intermediate and gage row lands. The inserts utilized are symmetrical inserts which are placed in a non-linear pattern on the cutters. While the Jones concept is a significant improvement over the conventional linear rows of inserts, the present invention provides further improvement over the aforesaid Jones method. The present invention utilizes non-symmetrical inserts which are placed in linear rows of insert holes, but which result in non-linear insert projections and increased bottom hole coverage, reduced tracking and gyration, and no sacrifice in rate of penetration.